This invention relates to a speech detector responsive to an input signal including a speech or voice signal as a desired signal for detecting presence and absence of the speech signal.
It has already been pointed out that a normal telephone conversation effectively utilizes only about 40% of time on unidirectionally transmitting a speech signal along a transmission line and uselessly wastes the remaining time. Thus, a utilization rate during which the transmission line is effectively utilized is very low in the normal telephone conversation. In order to raise the utilization rate, a speech transmission system has been proposed which can realize effective transmission of the speech signal by transmitting the speech signal only during presence thereof and, otherwise, any other data signals. A speech detector of the type described is used in such a speech transmission system to detect presence and absence of the speech signal.
A conventional speech detector monitors electric power of an input signal to determine presence of the speech signal when the monitored electric power becomes higher than a predetermined or fixed threshold level. Let an ambient noise or background noise be included, as an undesired signal, in the input signal in addition to the speech or desired signal. When the electric power of the input signal is monitored to be compared with the predetermined threshold level, it may always exceed the predetermined threshold level. As a result, the speech detector wrongly detects presence of the speech signal and brings about deterioration of the utilization rate. On the other hand, a higher threshold level gives rise to an interruption at the beginning of each talk or speech. In view of the circumstances, it is possible to adaptively vary a threshold level in response to a level of the undesired signal. However, the interruption at the beginning of each speech inevitably takes place when the level of the undesired signal is equal to or higher than a level of the speech signal.
In IEEE Transactions on Communications, vol. COM-26, No. 1, pp. 140-145 (January, 1978), P. G. Drago et al have proposed a digital dynamic speech detector which detects a speech signal by deriving an envelope of the speech signal to successively monitor relative variations of the envelope between two adjacent time instants. With this speech detector, it is difficult to correctly detect presence of the speech signal when each relative variation is narrow, such as vowels.
In U.S. Pat. No. 4,401,849 issued to Akira Ichikawa et al, a speech detecting method is disclosed which monitors partial auto-correlation coefficients determined in relation to a frequency spectrum of the input signal. The speech detecting method is disadvantageous in that the undesired signal will be erroneously detected as a desired signal when the undesired signal exhibits the partial auto-correlation coefficients which are similar to those of the desired signal.